1. Field of the Invention
The field of art to which the claimed invention pertains is solid-bed adsorptive separation. More specifically, the claimed invention relates to a process for the separation of ethylbenzene from a feed mixture comprising ethylbenzene and at least one xylene isomer which process employs a solid adsorbent and desorbent combination which effects selective removal of ethylbenzene from the feed mixture.
2. Background Information
It is well known in the separation art that certain crystalline aluminosilicates can be used to separate hydrocarbons species from mixtures thereof. In particular, the separation of normal paraffins from branched chained paraffins can be accomplished by using the type A zeolite which have pore openings from 3 to about 5 Angstroms. Such a separation process is disclosed for example in U.S. Pat. Nos. 2,985,589 and 3,201,491. These adsorbents allow a separation based on the physical size differences in the molecules by allowing the smaller or normal hydrocarbons to be passed into the cavities within the crystalline aluminosilicate adsorbent, while excluding the larger or branched chain molecules.
U.S. Pat. Nos. 3,265,750 and 3,510,423 for example disclose processes in which larger pore diameter zeolites such as the type X or type Y structured zeolites can be used to separate olefinic hydrocarbons.
In addition to separating hydrocarbon types, the type X or type Y zeolites have also been employed in processes to separate para-xylene from a feed mixture comprising para-xylene and at least one other xylene isomer by selectively adsorbing para-xylene over the other xylene isomers. Such processes are disclosed in U.S. Pat. Nos. 3,732,325; 3,997,620; 4,029,717; 4,031,155; and 4,255,607. These references disclose various adsorbent (including Ba and K exchanged X-type zeolites) and desorbent combinations which effect selectivity for xylenes over ethylbenzene, but none teach benzene as a desorbent material, optimum water content of the adsorbent, nor, of course, ethylbenzene as the extract component.
Ethylbenzene is used as a raw material in the production of styrene monomer. Ethylbenzene can be and is commercially produced from the alkylation of benzene with ethylene. The cost of and competing demands for necessary benzene and ethylene feed streams have, however, prompted new efforts to recover ethylbenzene from various C.sub.8 aromatic feed streams which already contain ethylbenzene. Such feed streams for instance, include C.sub.8 aromatic extracts produced by a typical solvent extraction process from a pyrolysis gasoline or from a naphtha which has been reformed with a platinum-halogen-containing catalyst. Additionally C.sub.8 aromatic cuts of hydrogenated pyrolysis naphthas or reformates prepared by fractionation without solvent extraction contain varying amounts of ethylbenzene.
Ethylbenzene can, of course, be separated from the xylene isomers by fractionation but because its boiling point is within about 4.degree. F. of that of para-xylene, the fractionation can be achieved only with the more intricate super-fractionators. Typical ethylbenzene fractionators contain 300 to 400 actual trays and require about a 25-50 to 1 reflux to feed ratio. The process of our invention therefore offers a competitive alternative to the separation of ethylbenzene by super-fractionation.
U.S. Pat. No. 3,943,182 discloses a process for extracting ethylbenzene from admixture with xylenes which employs a type X zeolite having only Group I-A ions at exchangeable cationic sites, and which contains about 0.02 to 2.5 wt. % water. Benzene is mentioned as a possible but not preferred desorbent.
The present invention is based on the surprising discovery that, in contradistinction to much of the teaching in the art, an X type zeolite adsorbent having Ba exchanged cations is selective for ethylbenzene when the adsorbent contains an amount of water within a specified range and when benzene is employed as the desorbent. Furthermore, the adsorbent having such composition is superior to the adsorbent of U.S. Pat. No. 3,943,182 for the separation of ethylbenzene from admixture with xylenes.